1. Field of the Invention
The present invention relates to a drive circuit for a lamp, and more particularly to inverter driving of a fluorescent lamp.
2. Related Background Art
FIG. 5 is a diagram showing the structure of a conventional fluorescent lamp drive circuit, and FIGS. 6A to 6G are timing charts showing signal waveforms of the conventional fluorescent lamp drive circuit shown in FIG. 5 and a fluorescent lamp drive circuit according to the present invention.
In FIG. 5, the fluorescent lamp drive circuit is the fluorescent lamp drive circuit of an image formation apparatus, e.g., a copy machine. The fluorescent lamp drive circuit is composed of a fluorescent lamp power supply circuit 1, a fluorescent lamp inverter control circuit 2 for controlling a fluorescent lamp inverter of the circuit 1, a fluorescent lamp 5 for exposing an original, a light quantity sensor 8 for detecting a light quantity of the lamp 5, a preheating transformer T2 for preheating filaments 6 and 7 of the lamp 5, a light modulation control circuit 10 of the lamp 5, a microcomputer 14 (to be referred as copy machine control microcomputer hereinafter) for controlling the copy machine acting as the image formation apparatus, a first source oscillation circuit 15, division circuits 16 to 20 for frequency-dividing a clock pulse generated from the circuit 15, and the like.
A fluorescent lamp inverter drive signal of a frequency F which is output from a triangular wave generation circuit 4 of the fluorescent lamp inverter control circuit 2 in the fluorescent lamp drive circuit structured as above and then supplied to a fluorescent lamp inverter drive circuit 3 is shown in FIG. 6A. A fluorescent lamp light modulation drive signal of a frequency f which is output from a pulse width modulation (PWM) circuit 13 of the light modulation control circuit 10 and then supplied to a base of a switching transistor Q3 of a diode bridge DB2 is shown in FIG. 6B. A current waveform flowing in the filaments 6 and 7 of the fluorescent lamp 5 is shown in FIG. 6G.
Conventionally, the fluorescent lamp inverter drive signal of the frequency F is often generated from a charge pump circuit by using an analog control IC (e.g., .mu.PC494 or the like). For this reason, blurring of a setting frequency or a frequency temperature characteristic is several percent or so, and a drift happens in the unit of several seconds or several minutes after the power supply of the copy machine is turned on. FIG. 6A shows an example that the frequency F of the fluorescent lamp inverter drive signal is drifted in the order of 83 kHz.fwdarw.85 kHz.fwdarw.87 kHz.
On the other hand, since the fluorescent lamp light modulation drive signal of the frequency f concerns a flip-flop circuit 11, an up/down counter 12, the PWM circuit 13, the copy machine control microcomputer 14, a not-shown CCD drive circuit and the like, such the signal is often generated by appropriately frequency-dividing the clock pulse generated from the first source oscillation circuit 15 with the division circuits 16 to 20. For this reason, as compared with the frequency F of the fluorescent lamp inverter drive signal, since there is hardly blurring of a setting frequency or a frequency temperature characteristic in the frequency f of the fluorescent lamp light modulation drive signal, such the setting frequency and the frequency temperature characteristic are constant.
FIG. 6G shows the current waveform which is obtained by synthesizing (i.e., AND) the fluorescent lamp inverter drive signal in FIG. 6A and the fluorescent lamp light modulation drive signal in FIG. 6B and flows in the filaments 6 and 7 of the lamp 5. It should be noted that the frequency F of the fluorescent lamp inverter drive signal shown in FIG. 6A is asynchronous with the frequency f of the fluorescent lamp light modulation drive signal shown in FIG. 6B. Further, since the frequency F drifts as described above, the number of pulses of the frequency 85 kHz of the fluorescent lamp inverter drive signal included in each one period (2.7 kHz) of the fluorescent lamp light modulation drive signal differs in each period as shown in FIG. 6G.
In the above-described conventional fluorescent lamp drive circuit, however, there are following drawbacks. That is, since the frequency F of the fluorescent lamp inverter drive signal and the frequency f of the fluorescent lamp light modulation drive signal may produce a beat, a substantial lighting current of the lamp 5 varies, whereby the light quantity also varies.
In addition, if a value of .vertline.F-f.times.N.vertline.=D (N: integer) is being within a predetermined range, unevenness or nonuniformity according to the above beat appears in a sub-scan direction on an image. For example, if it is assumed that the frequency f of the fluorescent lamp light modulation drive signal is 2.7 kHz and the frequency F of the fluorescent lamp inverter drive signal is 85 kHz to 87 kHz, a 32 integral multiple of 2.7 kHz is 2.7 kHz.times.32=86.4 kHz.
Here, if the frequency F is 86.3 kHz, the unevenness of 86.4 kHz-86.3 kHz=100 Hz appears on the image; if the frequency F is 86.41 kHz, the unevenness of 86.41 kHz-86.4 kHz=10 Hz appears on the image; and if the frequency F is 86.5 kHz, the unevenness/of 86.5 kHz-86.4 kHz=100 Hz appears on the image.
Basically, even if the frequency F has any value, the beat itself is produced. For example, if the frequency F is 85.9 kHz, the unevenness of 86.4 kHz-85.9 kHz=500 Hz appears; and if the frequency F is 86.9 kHz, the unevenness of 86.9 kHz-86.4 kHz=500 Hz appears.
However, if the frequency of the unevenness becomes equal to or larger than a predetermined value (e.g., .gtoreq.500 Hz), the unevenness on the image becomes invisible for human eyes. That is, in a case where the value of the frequency F and the value of the frequency f.times.N (N: integer) are closely coincided (i.e., synchronized) with each other, or in a case where these values are apparent from each other by a predetermined value or more, any unevenness does not appear on the image. On the other hand, if these values are not slightly coincided with each other, the unevenness corresponding to the difference between the frequencies not coincided appears on the image.